1. Field of the Invention
The present invention relates to a displacement measurement apparatus which is optimally applied for measuring or detecting a displacement of continuous measured points on a measured surface with respect to a reference plane, and specifically to a measurement apparatus which is capable of a displacement of a measured object by detecting an offset distance between a relative distance of an objective lens to a measured surface and a focal distance of the objective lens, by way of an astigmatism method.
2. Description of the Prior Art
As is generally known, in order to measure or detect a displacement or dimensions of a measured surface or a measured object, a displacement measurement apparatus which utilizes a laser beam and an objective lens, has been used. On such displacement measurement apparatus has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 4-366711 corresponding to Japanese Patent Application No. 3-141836, assigned to the same assignee of the present invention. FIG. 5 shows the prior art displacement measurement apparatus disclosed in the Japanese Patent Provisional Publication No. 4-366711. Referring now to FIG. 5, reference numeral 1 denotes a displacement detecting optical system which system detects an offset distance between a relative distance of an objective lens 5 to a measured surface 6 and a focal distance of the objective lens 5, by utilizing a reflection laser beam reflected from the measured surface 6. Reference numeral 16 denotes an actuator which is provided for moving the objective lens 5 in a direction of its optical axis 21 on the basis of an output signal produced by a four-split diode 7 employed in the optical system 1, so that the relative distance between the objective lens 5 and the measured surface 6 is adjusted to the focal distance of the objective lens. Reference numeral 20 denotes a linear scale which is coaxially aligned with an extension line 22 of the optical axis 21 of the objective lens 5. The linear scale 20 is integrally connected to the actuator 16 for motion therewith. The prior art displacement measurement apparatus operates as follows.
First of all, the optical system 1 detects an offset distance between the focal distance of the objective lens 5 and the relative distance of the objective lens 5 to the measured surface 6, by utilizing a reflection laser beam reflected from the measured surface 6. The four-split diode 7 produces an output signal representative of the offset distance. The actuator 16 is responsive to the output signal produced by the four-split diode 7, for moving the objective lens 5 in its optical axis 21 so that the relative distance of the objective lens 5 is adjusted to the focal distance of the objective lens 5. Since the linear scale 20 is integrally connected to the actuator 16 and coaxially aligned with the optical axis 21, the linear scale 20 can be moved by means of the actuator so that a displacement of the linear scale is in correspondence with a displacement of the measured surface 6. In this manner, the displacement of the measured surface 6 can be detected and measured by reading the displacement of the linear scale 20. As is generally known, the four-split diode 7 has a relatively narrow detectable region within which the four-split diode can detect the previously-noted offset distance. The prior art apparatus operates to move the objective lens 5 towards or away from the measured surface 6 at a constant speed when detecting a displacement of the measured surface 6. In case that the constant speed is set at a low speed, there is a possibility that it takes a long time until the objective lens 5 reaches to the detectable region of the four-split diode 7. In case that the constant speed is set at a high speed, there is a possibility that the objective lens 5 overruns the detectable region of the four-split diode 7.